The present invention relates to a method of making a lithium ion polymer battery having improved cycling life and self-discharge properties at a low manufacturing cost by way of using a specified gel polymer electrolyte composition gellable at room temperature.
Lithium secondary batteries have common structural features that include a cathode, an anode, an organic electrolyte and a lithium ion-permeable separator disposed between the electrodes. The electrical energy is generated by redox reactions occurring on the electrodes. The lithium secondary batteries are classified into two types depending on the kind of electrolyte used: a lithium ion battery which employs a liquid electrolyte; and a lithium ion polymer battery, a solid polymer electrolyte.
Lithium ion polymer batteries are generally preferred over lithium ion batteries because of no risk of liquid electrolyte leakage and the capability of shaping it into any form, e.g., an ultra-thin battery.
Accordingly, many efforts have been made to develope suitable gel polymer electrolyte compositions for use in a lithium ion polymer battery. Japanese Publication No. 11-283672 and Japanese Publication No. 11-283673 disclose a method of preparing a polymer battery, which comprises gelating an electrolytic solution(xe2x80x9cpre-gelxe2x80x9d solution) composed of polyethyleneglycol diacrylate, benzoyl peroxide and an organic electrolyte by heating at 60xc2x0 C. for 1 hour. U.S. Pat. No. 5,639,573 and U.S. Pat. No. 5,665,265 disclose a method of preparing a stacking polymer battery which comprises a two-phase polymer separator and electrodes, and in which a liquid electrolyte is permeated into the stack by heating at 90xc2x0 C.
However, the above methods require the step of heating the electrolytic solution at 60xc2x0 C. or higher, which results in polymer batteries having poor performance characteristics, e.g., low cycling life and high self-discharge properties due to the undesired by-products from a thermal cracking of salts contained in the electrolyte at elevated temperature, and high manufacturing cost due to heating equipments in the process.
Accordingly, it is an object of the present invention to provide a method of making a lithium ion polymer battery using a gel polymer electrolyte composition gellable at room temperature, which makes it possible to prepare a lithium ion polymer battery having improved cycling life and self-discharge properties.
In accordance with one aspect of the present invention, there is provided a method of preparing a lithium ion polymer battery comprising the steps of introducing a gel polymer electrolyte composition into a battery case comprising at least one electrode stack composed of a cathode, an anode and a separator, and allowing the composition to gel at room temperature.